A generic algorithm for constructing hierarchical representations of geometric objects Page: 1 of 9
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Submitted to 1996 IEEE Int'l Conf on Robotics and Automation
A Generic Algorithm for Constructing Hierarchical
Representations of Geometric Objects*
Patrick G. Xavier
Sandia National Laboratories, Albuquerque NM 87185-0951
OCT 1 1 1995
For a number of years, robotics researchers have
exploited hierarchical representations of geometri-
cal objects and scenes in motion-planning, collision-
avoidance, and simulation. However, few general
techniques exist for automatically constructing them.
We present a generic, bottom-up algorithm that uses
a heuristic clustering technique to produced balanced,
coherent hierarchies. Its worst-case running time
is O(N2logN), but for non-pathological cases it is
O(NlogN), where N is the number of input primitives.
We have completed a preliminary C++ implemen-
tation for input collections of 3D convex polygons and
3D convex polyhedra and conducted simple experiments
with scenes of up to 12,000 polygons, which take only
a few minutes to process. We present examples using
spheres and convex hulls as hierarchy primitives.
For a number of years, the robotics community has uti-
lized hierarchical geometric representations to avoid all
pairs comparisons in distance computations and col-
lision detection, with motion planning being the main
application. (For example, see [FT87, Qui94].) The
automatic generation of hierarchical geometric repre-
sentations, however, is still a research area, especially if
one wants to control properties such as spatial balance,
structural balance, coherence, and tightness.
*This research was supported by DOE Contract DE-ACO4-
94AL85000, and by the Laboratory Directed Research and Devel-
opment Office of Sandia National Laboratories.
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We present a practical generic algorithm for gener-
ating hierarchical representations. Our algorithm is
generic in that it can be applied to collections of any con-
vex primitive, e.g., a scene of composed of convex poly-
gons or polyhedra. The core of the algorithm is a greedy
bottom-up clustering technique that constructs a tree of
sets. Tuning parameters allow control of the balance
of desirable properties. A hierarchy of convex solids
is then constructed by walking the tree. Our algorithm
is practical in that is runs in time roughly O(N log N);
our preliminary C++ implementation takes less than five
minutes to construct hierarchical groupings from inputs
of over 10,000 polygons. Building a hierarchy of solids
takes seconds to minutes, depending on the solids and
the desired tightness of the geometrical approximation.
Geometric query algorithms (e.g., distance) frequently
take tree-structured representations as input. A typical
representation is a tree whose nodes each contain a ge-
ometric primitive, such as a sphere, convex polygon, or
convex polyhedron. The subtree rooted at a node repre-
sents the union of the primitives at its leaves. We require
that each node of a hierarchical geometric representa-
tion contain a conservative approximation, or wrapper,
of the object represented by its subtree.
A typical branch-and-bound algorithm to answer a
query about the geometric object represented by such a
tree does a partial traversal, making a local query about
the wrapper or primitive at each node it visits. Thus, a
optimal tree would minimize
where cl(a) is the cost of answering the local query to
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Xavier, P.G. A generic algorithm for constructing hierarchical representations of geometric objects, article, October 1, 1995; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc664081/m1/1/: accessed October 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.